The present invention relates generally to stroke sensors suitable for detecting a stroke position of a fluid-pressure cylinder or a spool position of a spool value, and particularly to stroke sensors of a type which includes a coil section to be excited by an A.C. signal and a magnetic substance or electric, conductor movable relative to the coil section. More particularly, the present invention concerns a self-induction-type stroke sensor that uses only primary coils excitable by a single-phase A.C. signal to produce A.C. output signals presenting amplitude function characteristics of a plurality of phases in response to a stroke position to be detected.
Hitherto, there have been known various types of position detector apparatus which are designed to detect a stroke position in a fluid-pressure cylinder or the like. Of these position detector apparatus, induction-type cylinder position detector apparatus using electromagnetic coils are known, for example, from Japanese Utility Model Publication No. HEI-2-26003. In each of such conventional induction-type cylinder position detector apparatus, raised and recessed portions or a pattern of magnetic substance or electric conductor are first formed on the surface of the piston rod, and then a given protective coating is applied to the piston rod surface having the raised and recessed portions or pattern of magnetic substance or electric conductor formed thereon. Specifically, the known apparatus are based on a phase-shift-type position detection scheme, where a plurality of primary coils are exited by plural-phase A.C. signals (e.g., sin xcfx89t and cos xcfx89t) shifted from each other in electric phase and signals induced on secondary coils by the primary coils are synthesized together to produce a single secondary output signal such that an electric phase difference, from the exciting A.C. signals, of output signals from secondary coils indicates a position of the Piston to be detected.
With the conventional induction-type cylinder position detector apparatus having the piston rod surface formed to have raised and recessed portions or pattern of magnetic substance or electric conductor, however, manufacturing and processing of the piston rod is very cumbersome and time-consuming. Further, in this case, each piston rod must be manufactured and processed on a special order basis. For these reasons, a same position detector apparatus can not be applied to cylinders of various different types and sizes. Further, even with the processed piston rod having undergone the given coating process, the coating tends to peel off, after several yearsxe2x80x2 use, due to sliding wear caused by repeated reciprocative movement of the piston rod, which would lead to poor durability of the piston rod.
To avoid the disadvantages of the conventionally-known induction-type cylinder position detector apparatus, a more sophisticated cylinder stroke position detector apparatus has been proposed, for example, in Japanese Patent Laid-open Publication No. HEI 10-153203. The proposed cylinder stroke position detector apparatus is characterized in that the piston rod has an inner space for entry therein a sensor structure so that the need for applying particular processing to the peripheral surface of the piston rod can be eliminated and various components of the detector apparatus can be readily shared between cylinders of various different sizes. Also, the proposed cylinder stroke position detector apparatus thus arranged can have increased durability to reliably avoid the possibility of undesired sliding wear. The sensor structure provided in the inner space of the piston rod includes both primary coils and secondary coils, and the coil structure has much room yet to be improved.
Namely, because the coil structure of the inductive-type position sensor employed in the conventional cylinder stroke position detector apparatus requires both the primary and secondary coils, the necessary number of the component parts in the apparatus significantly increases so that significant limitations would be encountered in reducing the manufacturing cost and size of the detector apparatus. There is also known a position detector of a type that is designed to measure self-inductance of the exciting coils and can reduce the necessary number of the coils; however, a phase variation amount corresponding to displacement of an object to be detected in this position detector can be obtained only within a very narrow range, and thus the position detector presents the problems that the measurement of the phase variation amount is, in fact, difficult and the detecting resolution is very low, so that it can never be suitable for practical use.
It is therefore an object of the present invention to provide a self-inductance-type stroke sensor which is compact in size and simple in structure, provides for a wider detectable stroke range and also achieves high-resolution detection even when displacement of an object to be detected is very minute.
In order to accomplish the above-mentioned object, the present invention provides an improved stroke sensor for detecting a stroke position of a movable member linearly movable relative to a main body within a space formed in the main body, which comprises: a sensor member having one end supported in a cantilever fashion adjacent a predetermined end of the main body and having another end extending into the space of the main body, the movable member having an inner space formed therein to permit entry of the sensor member into the movable member; a coil section having a plurality of coil segments excitable by a predetermined A.C. signal and sequentially arranged along a direction of linear movement of the movable member, the coil section being provided on one of the sensor member and an inner peripheral wall of the movable member defining the inner space; and a magnetism-responsive substance provided on other of the sensor member and the inner peripheral wall of the movable member defining the inner space in such a manner that the magnetism-responsive substance is movable relative to the coil section. Relative positions of the magnetism-responsive substance and the coil section vary in accordance with a stroke position of the movable member, in response to which respective inductance of the coil segments are caused to vary in such a manner that during movement of the magnetism-responsive substance from one end to the other of a particular one of the coil segments, a voltage across the particular coil segment is caused to progressively decrease or increase.
In the case where the present invention is applied as a stroke sensor for a fluid-pressure cylinder, the above-mentioned xe2x80x9cbodyxe2x80x9d corresponds to the cylinder body, and the above-mentioned xe2x80x9cmovable memberxe2x80x9d corresponds to the piston rod. In this case, the sensor member is supported at one end, in a cantilever fashion, adjacent a predetermined end of the cylinder body in such a way that its other end extends into the space of the cylinder body. Further, the piston and piston rod, i.e. the movable member, has an inner space formed therein to permit entry of the sensor member into the movable member. Such a structure is similar to what is disclosed in Japanese Patent Laid-open Publication No. HEI 10-153203 discussed above, which, as described therein, affords various benefits, such as simplification, enhanced durability, compactness and wider applications of the structure. Here, the stroke position detection is effected by detecting, in accordance with the induction principles, relative positions of the coil section and magnetism-responsive substance provided for relative displacement to the coil section, on the basis of output voltages corresponding to inductance variations of the individual coil segments that occur in response to changing relative positions of the coil section and magnetism-responsive substance. In the case where the present invention is applied as a spool valve position detector apparatus, on the other hand, the above-mentioned xe2x80x9cmovable memberxe2x80x9d corresponds to the spool.
As an example, the coil section is provided on the sensor member, in which case the magnetism-responsive substance is provided on the inner peripheral wall of the movable member defining the inner space. If the material of the movable member itself comprises a predetermined magnetism-responsive substance, then the inner peripheral wall of the movable member defining the inner space itself functions as the above-mentioned magnetism-responsive substance, which therefore can eliminate a need for providing a separate magnetism-responsive substance other than the inner peripheral wall. In case the material of the movable member itself does not comprise a predetermined magnetism-responsive substance, such a predetermined magnetism-responsive substance has to be separately provided on the inner peripheral wall of the movable member defining the inner space. Conversely, the coil section may be provided on the inner peripheral wall of the movable member defining the inner space provided on the sensor member and the magnetism-responsive substance may be provided on the sensor member, because the positional relationship between the coil section and the magnetism-responsive substance can be reversed without involving inconveniences.
Typically, the magnetism-responsive substance may comprises at least one of a magnetic material and an electric conductor. In the case where the magnetism-responsive substance comprises a magnetic material, as the magnetism-responsive substance moves closer to or deeper into any one of the coil segments, the self-inductance of the coil segment increases, and thus the voltage across (i.e., between the opposite ends of) the coil segment increases progressively during displacement of the tip of the magnetism-responsive substance from one end to the other of that coil. Here, because the plurality of coil segments are arranged in series along the displacement direction of the movable member that is the object of the position detection, progressively increasing (or progressively decreasing) variations in the respective voltages of the coil segments will occur sequentially as the magnetism-responsive substance moves relative to the coil section in response to the displacement of the object to be detected. Using combinations of the progressively increasing (or progressively decreasing) variations in the respective between-terminal voltages of the coil segments while regarding the voltage variations as variations in partial phase ranges of predetermined cyclic functions, there can be produced a plurality of A.C. output signals presenting amplitudes of predetermined cyclic function characteristics in accordance with the stroke position of the object to be detected. Namely, the plurality of A.C. output signals presenting amplitudes of predetermined cyclic function characteristics in accordance with the stroke position of the object to be detected can be produced by taking out the respective between-terminal voltages of the individual coil segments and performing addition and/or subtraction between the taken-out voltages of the coil segments to thereby provide combinations of the between-terminal voltages.
For example, a progressively-increasing variation curve of the voltage across one of the coil segments, which takes place during the movement of the magnetism-responsive substance from one end to the other of the coil segment, can be likened to a functional value variation within a 0xc2x0-90xc2x0 range of the sine function. The progressively-increasing variation curve can be converted to a variation curve progressively decreasing from a predetermined level, by inverting the amplitude values to corresponding negative values and subjecting the inverted amplitude values to a voltage shift operation to add thereto predetermined levels (offset levels). Such a progressively-decreasing variation curve can be likened, for example, to a functional value variation within a 90xc2x0-180xc2x0 range of the sine function. Thus, by performing appropriate addition and/or subtraction between the between-terminal voltages as necessary, the progressively-increasing variations of the between-terminal voltages sequentially occurring in serially-arranged four coil segments can be likened respectively to functional value variations within the 0xc2x0-90xc2x0 range, 90xc2x0-180xc2x0 range, 180xc2x0-270xc2x0 range and 270xc2x0-360xc2x0 range of the sine function. The sloping directions of the curves in the individual ranges and voltage shift offset levels can be controlled as necessary through appropriate analog arithmetic operations. In this way, there can be produced a first A.C. output signal presenting an amplitude of the sine function characteristics in accordance with the position of the object to be detected and a second A.C. output signal presenting an amplitude of the same function characteristics which is shifted in phase from the sine function by 90xc2x0.
As a preferred implementation, there may be produced two A.C. output signals presenting amplitudes of the sine and cosine function characteristics, respectively, in accordance with the position of the object to be detected. Generally speaking, if the position of the object to be detected is denoted by an angle xcex8, the A.C. output signal of the amplitude presenting the sine function characteristics can be represented by xe2x80x9csin xcex8 sin xcfx89txe2x80x9d, while the A.C. output signal of the amplitude presenting the cosine function characteristics can be represented by xe2x80x9ccos xcex8 sin xcfx89txe2x80x9d. These signals are similar in form to output signals from conventional position detector apparatus commonly called xe2x80x9cresolversxe2x80x9d and can be used extremely usefully. For example, the present invention may include an amplitude-to-phase conversion section that receives the plurality of A.C. output signals generated by the analog arithmetic operation circuit and detects, from a correlation between the amplitude values in the plurality of A.C. output signals, particular phase values in the predetermined cyclic functions defining the amplitude values, to generate data indicative of the position of the movable member to be detected on the basis of the detected particular phase values.
In the case where the magnetism-responsive member comprises a good electric conductor such as copper, the self-inductance of the coil segments would decrease due to eddy-current loss, so that during movement of the magnetism-responsive substance from one end to the other of a particular one of the coil segments, a voltage across the particular coil segment is caused to progressively decrease. In this case too, the position detection can be effected in the same manner as the above-described case. The magnetism-responsive substance may be provided as a hybrid type which comprises a combination of the magnetic material and electric conductor. As another implementation, a permanent magnet may be used as the magnetism-responsive substance, and the coil segments may include a magnetic core. In such a case, as the permanent magnet, functioning as the magnetism-responsive substance, approaches any one of the coil segments, a portion of the magnetic core having come close to the permanent magnet gets magnetically saturated or supersaturated, which results in a progressive drop in the between-terminal voltage of the coil segment during displacement of the magnetism-responsive substance, i.e. the permanent magnet, from one end to the other of the coil segment.
Thus, the present invention arranged in the above-mentioned manner only requires primary coils and can eliminate a need for secondary coils, so that the present invention can provide a position detector apparatus which is compact and simple in structure. Further, the plurality of coil segments are arranged in series along the displacement direction of the object to be detected and variations in characteristics of the coil segments, i.e. progressively increasing (or progressively decreasing) variations in the respective between-terminal voltages of the coil segments, occur sequentially as the tip of the magnetism-responsive substance moves from one end to the other of any one of the coils. Thus, by taking out the respective between-terminal voltages of the coil segments and then combining them through addition and/or subtraction thereof, there can be readily produced a plurality of A.C. output signals that present respective amplitudes of predetermined cyclic function characteristics (e.g., two A.C. output signals that present respective amplitudes of the sine and cosine function characteristics) in response to a current position of the object to be detected; thus, a wider available phase angle range can be provided by the present invention. For example, the present invention permits detection over the full 0xc2x0-360xc2x0 phase angle range, as described above. Further, by detecting, from a correlation between the amplitude values in the plurality of A.C. output signals, phase values in the predetermined cyclic functions (e.g., sine and cosine functions) defining the amplitude values, the present invention achieves high-resolution detection even when the displacement of the object to be detected is very minute.